Insect damage to crops is a significant factor in decreasing crop yield and considerable effort is expended annually in controlling insect populations to minimise or prevent crop damage. Although chemical insecticides have played an important role in controlling insect populations, increasing resistance to these insecticides and environmental concerns have rendered the use of chemical insecticides less appealing.
Resistance Management Strategies (RMS's) have been developed in order to minimise insecticide resistance and usually involve the administration of more than one insecticide. However, such strategies are significantly more expensive, are time consuming and do not address the environmental concerns.
The insect genera Heliothis and Helicoverpa comprise economically important pests which attack a range of crops including cotton, tobacco, maize, sorghum, sunflower, soybean, chick peas and other pulses, rapeseed, canola, ground nuts, lucerne and many horticultural crops such as cut flowers and tomatoes and other vegetables (Fitt, 1989).
Helicoverpa (Heliothis) armigera and Helicoverpa (Heliothis) punctigera (Lepidoptera: Noctuidae) (hereinafter referred to as H. armigera and H. punctigera) cause crop damage which is economically significant to agricultural industry. Whilst H. punctigera is the more abundant species in Australia, H. armigera is considered the more problematic of the two because it has a history of developing resistance to the chemical insecticides which have been used in its control (Fitt, 1989). Helicoverpa punctigera, however, does not appear to have the same propensity for developing resistance to insecticides.
Heliothis virescens and Helicoverpa zea (hereinafter referred to as H. zea) are important pests of crops throughout North America and especially crops in the U.S.A. Whilst both species have developed resistance to some insecticides, (Fitt, 1989) they are distinguished by the fact that Heliothis virescens may display up to 150 fold resistance to synthetic pyrethroids whereas H. zea shows no detectable resistance to these compounds (Elzen et al., 1992). Synthetic pyrethroids are therefore more efficacious for control of H. zea than for Heliothis virescens.
Heliothis virescens is congeneric with Heliothis (Neocleptria) punctifera whereas H. zea is congeneric with Heliocoverpa armigera and Helicoverpa punctigera.
Resistance Management Strategies developed for H. armigera and H. punctigera have involved a rotation of pyrethroids, endosulfan and other insecticides to reduce exposure to endosulfan and pyrethroids. This has been necessary due to the increasing resistance to synthetic pyrethroids and endosulfan. Despite these RMS's, the frequency of resistant individuals has been steadily increasing (Forrester, 1990).
Helicoverpa armigera and H. punctigera at their egg and neonate stages are morphologically indistinguishable, whilst their respective larvae can only be distinguished by experts. A similar situation exists for Heliothis virescens and H. zea. Current RMS therefore make assumptions based on historical records of the relative abundance of the two species, averaged over wide geographical distances and several seasons. These assumptions necessarily entail some degree of inaccuracy, the consequence of which is that, in some cases, expensive alternative insecticides may be applied to H. punctigera, whilst in other cases, there may be multiple applications of pyrethroids to populations of H. armigera. In the latter case, the pyrethroid applications may be ineffective and will generally encourage the emergence of insecticide resistance. Similar problems also arise with Heliothis virescens and H. zea. Such situations may lead to unnecessarily expensive insect control for farmers. For example, insecticides such as pyrethroids which are adequate for species without a propensity to develop resistance are comparatively cheap insecticides. These may cost the farmer 3 or 4 times less to apply than those insecticides which are applied to insect species with a tendency to develop insecticide resistance. There is a need, therefore, to develop a convenient method to determine the species composition of egg lays, neonates or older larva which may have survived an insecticide application in the field. Such a method would be particularly useful for routine use by crop growers/farmers, agronomists and/or agricultural scientists so that insecticide applications can then be tailored to the composition of the Heliothis or Helicoverpa population actually present.
In work leading up to the present invention, the present inventors have found an immunologically reactive molecule (IRM) that distinguishes between insect species or subspecies. The IRM is capable of binding to a target molecule in at least one species or subspecies but does not react with at least one other species or subspecies. This provides a basis for distinguishing between or identifying insect species or subspecies.